1. Field of the Invention
The present invention relates to method and apparatus for stacking documents and more particularly relates to a restacking system in a copier having a recirculating document handler.
2. Background
As xerographic and other copiers have become more automated, the need for high speed yet reliable methods for handling both copy sheets and original documents to be copied has increased. The capability to copy both sides of an original complicates and increases the document and copy sheet handling requirements of the copier. In automatic duplex copying the document originals must be transmitted to a copier platen and then restacked so the opposite document side may be copied.
So called recirculating document handlers (RDHs) perform the complex function of manipulating the document original in a controlled manner past the platen for copying. In a typical recirculating document handler, documents are placed by the copier user in normal collated order in a copier stacking tray and then sequentially fed from the bottom of that tray to a conventional platen where they are imaged onto a photoreceptor. After one side of the original has been copied, the document is returned to the top of the stacking tray via a return feed path.
Moving the document around this feed path at high speeds becomes difficult if the copier is to handle a variety of sizes and weights of papers. As copier flexibility is increased the complexity of the recirculating document handler is also increased and in particular the restacking of documents in the paper tray becomes difficult.
The problem of landing a series of documents in a copier stacking tray is more than a problem in free fall dynamics. The trajectory of the document must be controlled so that the document is not damaged and so that the document hits a landing area and stops in approximate position registration so that it can be recirculated past the platen for duplex copying.
3. Prior Art
A February 1978 research disclosure article No. 16,659 by Boase, E., et al., entitled "Recirculating Document Feeder" addresses the problem of landing the document in the restack tray. The apparatus disclosed in that article includes a vacuum belt traveling beneath both a vacuum chamber and a pressure chamber which are connected by a narrow passage. Both chambers have ports facing the belt. As a document is delivered to the belt, the document seals the ports and is held to the belt by a vacuum. The document is held against the belt until its trailing edge uncovers the vacuum port. When this occurs, the vacuum chamber communicates with the atmosphere so the amount of air delivered from the pressure chamber to the vacuum chamber through the passageway is diminished. The pressure at the port still in contact with the document is accordingly increased, "gently" separating the document from the belt and blowing the document downward into a hopper or restack tray.
A different kind of paper stacking technique is disclosed in U.S. Pat. No. 3,918,701 to Lee entitled "Sheet Fed Paper Tray Paper Retaining Assembly". That patent shows the use of an assembly which includes a deflector which contacts a sheet fed into the paper tray and deflects it towards the bed of the paper tray. After the paper is deflected it is stopped by a second member and stacked in the tray by the force of gravity.
U.S. Pat. No. 3,820,779 to Bishop entitled "Sheet Delivery Apparatus" discloses apparatus for feeding cards or the like to a stack. Apparatus constructed in accordance with this patent includes so called "ejector strips" which move into frictional engagement with the card and physically separate it from a system conveyor belt. Once the separation occurs gravity attracts the card to a stack position beneath the conveyor.
The apparatus disclosed in U.S. Pat. No. 4,129,295 to Hori et al also deflects moving documents in their path to a paper restack tray. A guide element positioned above the tray intercepts documents discharged along an upward path and guides the documents down into the stacking tray.
Both the forced air and the deflector techniques for sheet restacking have deficiences. While the forced air technique "gently" blows the document to the restack tray, that document is experiencing a variety of forces which can be described as unpredictable at best. If the document is to be recirculated in a reliable manner the sheets must be restacked and not merely piled one on another in a haphazard manner. The research disclosure stacking technique suffers from a second deficiency perhaps more serious than the first. In high speed recirculating documents it is simply not feasible to wait for the document to be blown "gently" onto the restack tray in an uncontrolled manner.
Those systems with deflectors will restack at high speeds but at the expense of damage to the document. The deflector contacts every document's leading edge as the document enters the restack tray. As the document speed is increased to improve copier speed the impact between deflector and leading edge tends to dent the leading edge, especially for light weight documents. Modifying the '701 deflector by using a lightweight flexible material has been tried with only slight improvement in maintaining document integrity.